The present invention relates to a collapsible core and a method for producing the collapsible core feasible for high pressure casting such as a die casting and squeeze casting (cast forging).
In an ordinary high pressure casting such as die casting etc., a metallic core is used to increase its mechanical strength so as to withstand high pressure. However, it would be rather difficult to remove such metallic core from a casted material after casting, particularly in case of an intricate cores. Therefore, the core having relatively simple configuration is only available for the die casting, and therefore, casting products undergo restrictions in terms of its variety in shape.
To overcome this problem, a collapsible core has been proposed instead of the metallic core. However, such collapsible core would be detrimental to mechanical strength, surface peneteration with molten metal such as aluminum, collapsibility after foundry, and quality of casting surface of the product. Therefore, the conventional collapsible core would be lack of a commercial feasibility.
Various researches and development have been made to overcome the drawbacks in the conventional collapsible core. It would be rather difficult to summarize such R & D, but fundamentally, coating layers are formed on an external surface of a core body, and the tequniques are disclosed in Japanese Patent Publication Nos. 57-59013 and 60-15418 and Japanese Laid Open Patent Publication No. 59-45054.
According to the publications 57-59013 and 60-15418, dual layers are provided over the core body. That is, a first layer "b" formed of refractory material mixed with resin is provided over the surface of the core body, and a second layer "a" formed of mica is provided over the first layer as shown in FIG. 1. In this construction, since bonding strength between the first and second layers is insufficient, the second layer "a" may be washed out by turbulent melted metal flowing at high speed in the die casting machine. As a result, the core undergoes metal penetration.
Further, the first layer includes resin having high resistance to heat, and the sand core is subjected to deep penetration with the resin. Therefore, after foundry, the core may not be easily collapsible, and the resin must be heated at high temperature for its decomposition, i.e., so called sand baking step is required. However, in the die casting method which provides high speed casting with turbulent molten metal flow, blisters on casting may occur due to the sand baking step. The blisters on casting causes degradations in external appearance, dimensional accurcy, and shape of the casted product, and therefore, the sand baking step should not be applied.
Accordingly, the conventional collapsible core is only available for special type of casting method where molten metal passes through a gate at extremely low flowing velocity. Therefore, it would be almost impossible to widely use the conventional collapsible core in various casting manners.